At present, the light source widely used in the backlight module is light-emitting diode (LED). The LED light source has the following two outstanding characteristics. First, the LED is a spot illuminant and the intensity of light emitted from its light-emitting center is the strongest, thus the intensity of light emitted from the light output surface tends to gradually decrease from the center to both sides. Second, since the light-emitting range of the LED illuminant is limited, and the light-emitting angle thereof is usually greater than 90° and smaller than 120°, the range of light emitted from the light output surface is similar to a fan shape.
As shown in FIG. 1(a), the LED illuminant 12 will generate a light-emitting range as shown by the arrow directions in the figure. In the prior art, in order to guide light emitted by the LED illuminant 12 into a light guide plate 20 to the greatest extent, while avoiding light loss of the incident light due to the refraction of air in a gap between the LED illuminant 12 and the light input surface 201 of the light guide plate 20, the light input surface 201 of the light guide plate 20 is usually set in direct contact with the light output surface (not shown) of the LED illuminant 12. Due to the inherent limitation of the light-emitting angle of the LED illuminant 12, after light emitted from the LED illuminant 12 enters the light guide plate 20, an area with bright light (i.e. bright spot 01) will be formed in front of the light output surface of the LED illuminant 12, while an area with dark light (i.e. dark spot 02) will be formed between two adjacent LED illuminants. Such alternation of brightness and darkness is called hot spot phenomenon. When the LED illuminants are arranged at equal intervals, such phenomenon of alternation of brightness and darkness is more serious. The hot spot phenomenon would cause the homogeneous brightness to decrease at the edges of a display device, thereby affecting the quality of the display picture.
To solve the problem, the purpose of alleviating the hot spot phenomenon is typically achieved in the prior art by increasing the number of LED illuminants or widening the width of the frame. However, increasing the number of LED illuminants would significantly increase the cost of the backlight light source and result in difficulty in heat dissipation of the LED illuminants, and widening the width of the frame would reduce the effective visible area of the display device and deteriorate the viewing effect. Moreover, frames that are too wide are difficult to meet the requirement of being light and thin on current display devices.